Callout request messages (callout requests) are requests made by an application program running on a data processing system to external application programs for data and/or services. The outgoing callout request messages are usually managed by the data processing system hosting the requesting application programs. A link to an external application program is generally established when an instance of an external application program notifies the host system that the external application is ready to accept callout requests. Although there may be multiple links to the host system, one link is generally active while other links are inactive at a particular time. The callout requests are generally sent one at a time over the active link, and further transmissions are generally queued until the host system receives either a callout response message to the request or an acknowledgement of receipt for the prior callout request.